1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, and more particularly to an image forming apparatus in which an electrostatic latent image is visualized by use of a two-component developer including toner and magnetic particles.
2. Discussion of the Background
An image forming apparatus in which an electrostatic latent image on an image carrier is developed with a two-component developer has been widely used. In this type of image forming apparatus, a magnetic brush is formed by magnetically adsorbing a two-component developer including toner and magnetic particles onto the surface of a developer carrier. An electrostatic latent image formed on an image carrier is developed with toner in the magnetic brush that is transferred from the developer carrier to the electrostatic latent image on the image carrier under the influence of a development electric field.
In an image forming process generally performed in an image forming apparatus that is not limited to an image forming apparatus using a two-component developer, an image is transferred onto a transfer sheet by a transfer device, and the transferred image on the transfer sheet is fused by heat and is fixed onto the transfer sheet by applying pressure to the image in a heat fixing device (hereafter referred to as a “heat fixing process”). The image on the transfer sheet is formed from substances including at least black particles, that is, so-called toner.
The electric power consumed when a heat fixing device fuses an image by heat constitutes a majority of the electric consumption in an image forming apparatus. Recently, demands for reduction in the electric consumption in an image forming apparatus have increased in view of the need for energy savings. To satisfy these demands, it is required that an image should be fused and fixed at lower temperatures.
For the above-described reasons, a toner that can be fused at lower temperatures (hereafter referred to as a low-temperature fixing toner) has been under development in recent years. In one background heat fixing process, the lower limit of fixing temperature was generally in a range of about 150° C. to about 170° C. On the other hand, in another background heat image fixing process, the lower limit of fixing temperature is set to be not greater than 150° C. To achieve the desired reduction in electric consumption, the low-temperature fixing toners have been used.
However, the low-temperature fixing toner generally tends to have a strong adhesion force. Therefore, the low-temperature fixing toner gradually fixes to a developing sleeve in a developing device, thereby deteriorating the developing ability of the developing sleeve. The fixing of toner to the developing sleeve often occurs at an area of the developing sleeve which faces a non-image portion of an image carrier in a developing region.
FIGS. 1A and 1B are schematic views for explaining one mechanism for the adhering of toner to the surface of a developing sleeve in an image forming apparatus using a two-component developer including toner and magnetic particles. Specifically, FIG. 1A illustrates a condition when a developing sleeve 109 faces an image portion of a photoreceptor 101 serving as an image carrier. FIG. 1B illustrates a condition when the developing sleeve 109 faces a non-image portion of the photoreceptor 101. In this image forming apparatus, a toner moves by a potential difference between the developing sleeve 109 and the photoreceptor 101.
For example, when a toner is negatively charged, the potential of the surface of the photoreceptor 101 is −950V, and the voltage of, for example, −600V having a polarity equal to that of the toner is applied to the developing sleeve 109. When an exposure device irradiates the surface of the photoreceptor 101 with a laser beam, the potential of the image portion of the photoreceptor 101 is changed to about −100V. On the other hand, the potential of the background portion of the photoreceptor 101 is maintained at −950V.
In the above-described conditions, as illustrated in FIG. 1A, because an electric field between the developing sleeve 109 and the image portion of the photoreceptor 101 is directed from the image portion of the photoreceptor 101 charged at −100V to the developing sleeve 109 charged at −600V as indicated by an arrow (A) in FIG. 1A, the negatively charged toner moves in a direction indicated by an arrow (B) in FIG. 1A and is adhered to the image portion of the photoreceptor 101.
On the other hand, referring to FIG. 1B, because an electric field between the developing sleeve 109 and the non-image portion of the photoreceptor 101 is directed from the developing sleeve 109 charged at −600V to the non-image portion (i.e., the background portion) of the photoreceptor 101 charged at −950V as indicated by an arrow (C) in FIG. 1B, the negatively charged toner moves in a direction indicated by an arrow (D) in FIG. 1B and is adhered to the developing sleeve 109.
As illustrated in FIGS. 1A and 1B, the amount of toner adhered to the surface of the developing sleeve 109 in an area of the developing sleeve 109 which faces the non-image portion of the photoreceptor 101 is much greater than that of toner adhered to the surface of the developing sleeve 109 in an area of the developing sleeve 109 which faces the image portion of the photoreceptor 101.
In the case of background toner (not low-temperature fixing toner), even when the toner adheres to the surface of a developing sleeve, the toner is easily removed from the developing sleeve when the toner is rubbed. However, when a low-temperature fixing toner is adhered to the surface of a developing sleeve, the low-temperature fixing toner is not easily removed from the developing sleeve and is fused with time. As a result, the low-temperature fixing toner tends to be fixed onto the developing sleeve.
The above-described toner that is fixed onto the surface of the developing sleeve forms an insulating layer on the surface of the developing sleeve. This insulating layer obstructs the formation of the developing electric field, thereby deteriorating the developing performance of the developing sleeve.
Therefore, it is desirable to provide an image forming apparatus that can form a stable quality image while preventing the fixing of toner to a developing sleeve even when a low-temperature fixing toner is used.